Along with advancement of information and communications technology, various types of radio communication technologies are being developed. Among others, wireless local area network (WLAN) technology, based on radio frequency technology, enables users to connect to the Internet in homes and businesses, or in particular service areas by using portable terminals such as personal digital assistants (PDAs), laptop computers, portable multimedia players (PMPs), etc.
IEEE 802.11n is a more recent technology standard for WLAN introduced to overcome low communication speed, which has been considered as a weak point of WLAN. IEEE802.11n has been introduced to improve network throughput and re-liability and extend operating range of a wireless network. More specifically, IEEE 802.11n supports high throughput (HT), where a maximum data processing rate is more than 540 Mbps. Also, IEEE 802.11n builds on multiple inputs and multiple outputs (MIMO) technology which employs multiple antennas for both of a transmitter-side and a receiver-side to minimize transmission error and optimize data rate.
As WLAN is widely accepted and applications based on WLAN are fast expanding, people have started to look for a new WLAN system capable of supporting a higher data processing rate than that supported by IEEE 802.11n. A next generation WLAN system supporting very high throughput (VHT) succeeds the IEEE 802.11n WLAN system; and the next generation WLAN system is one of IEEE 802.11 WLAN systems newly proposed these days to support data processing rate of more than 1 Gbps at a MAC service access point (SAP).
The next generation WLAN system supports data transmission based on a multi-user multiple input multiple output (MU-MIMO) method where multiple non-AP stations (STAs) access a channel simultaneously to utilize radio channels efficiently. According to MU-MIMO method, an AP can transmit a frame simultaneously to one or more STAs associated through MIMO-pairing.
Multiple STAs associated with an AP through MU-MIMO pairing can have the respective capabilities. Depending on the type, purpose, channel conditions, and the like of individual STA, bandwidth, modulation coding scheme (MCS), forward error correction (FEC), etc. can be varied.
According to MU-MIMO transmission scheme, a transmitter can transmit data to MU-MIMO paired multiple receivers through at least one or more spatial streams. At this time, a channel between the transmitter and a first receiver; and a channel between the transmitter and a second receiver can interfere with each other. In this way, channel interference between a transmitter and a receiver can block normal data transmission and reception, which may directly deteriorate the overall processing rate of a WLAN system. Therefore, to improve processing rate of a WLAN system supporting MU-MIMO transmission scheme, it is desirable to provide feedback of optimized modulation and coding scheme (MCS) recommendation sequentially by taking account of interference between different channels from each other. If multiple receivers provide feedback for a transmitter sequentially, a link adaptation method should be taken into account, including a method of determining the order of providing feedback among the receivers, a method of selecting a receiver for transmitting feedback, etc.